Xerographic development process



June 2, 1959 J. J. WALKER XEEOGEAPHIC DEVELOPMENT PROCESS Filed July 13,1955 FIG.I

I: n INVENTOR.

IG 2 JAMES J. WALKER A 'ATTORNEY' United States Patent 1 2,889,234XEROGRAPHIC DEVELOPMENT PROCESS James J. Walker, Columbus, Ohio,assignor, by mesne assignments, to Haloid Xerox Inc., Rochester, N.Y., a

corporation of New York Application July 13, 1955, Serial No. 521,668 1Claim. (Cl. 117-17.5)

This invention relates to a method and apparatus for the development ofelectrostatic latent images.

In xerography it is usual to form an electrostatic latent image on asurface. One method of doing this is to charge a photoconductiveinsulating surface and then dissipate the charge selectively byexposures to a pattern of activating radiation. Other means of formingelectrostatic latent images are set forth in U.S. 2,647,464 to lames P.Ebert and U.S. 2,576,047 to Roland M. Schatfert. Whether formed by thesemeans or any other, the resulting electrostatic charge pattern isconventionally utilized by the deposition of an electroscopic materialthereon through electrostatic attraction whereby there is formed avisible image of electroscopicparticles corresponding to theelectrostatic latent image. Alternatively, the electrostatic chargepattern may be transferred to an insulating film and the electroscopicparticles deposited thereon to form the visible image. In any case, thisvisible image in turn may be transferred to a second surface to form axerographic print.

It is at once evident that this process has exceptional flexibility andlends itself to a limitless number of end uses. On such application,which is particularly challenging in the strictness of its requirements,is the transfer of the visible image formed by the electroscopicparticles to a conductive plate and fixing the particles to the plate toform thereby a lithographie plate. Such plates vary in size from thedimensions of a postage stamp up to those used in making maps, whichplates are generally 2x3 in size. Furthermore, irrespective of the sizeof the image developed, an extremely quality of resolution is requiredthroughout, often as high as 150 lines per inch or better.

One particularly troublesome defect accompanying the transfer of thevisible image formed by the electroscopic particles to the conductiveplate is termed hollow lines. A line segment appearing on the originalas a solid line, on transfer to the conductive plate, appears as twovery narrow parallel lines with a white or unprinted area in between.Because of this appearance the defect has been given the name of hollowlines. A most unusual feature of this defect is that the visible imageformed on the photoconductive insulating surface by the depositedelectroscopic particles reproduces the solid lines of the origi-'eliminates this diltculty.

The developing material commonly used in xerography consists of a finelypulverized pigmented resin powder called a toner mixed with a coarsergranular material called a carrier to which the powder loosely adheresby the electrostatic charges generated by contact between the powder andthe granular carrier. When the developer mixture has tumbled over theexposed xerographic plate, as by tilting the plate to different angleswhile in contact with the developer mixture, the powder particles areattracted to the image from the granular material to produce a visiblepowder image. This method of development is known as ,cascadedevelopment and is set forth in U.S. 2,618,552. Applicant has now foundthat if, in this basic process of cascade development, the developingmaterial-rather than being permitted to ow in uniform parallel linesacross the surface of the xerographic plate-is made to follow a zig-zagcourse while cascading over the surface carrying the electrostaticlatent image, the resulting image formed by deposition of the toner onthe electrostatic latent image conforms faithfully to the original to bereproduced without loss of detail and, furthermore, an image developedin this way, on transfer to a conductive plate, is free from hollowlines.

By the term xerographic plate as used herein is meant, of course, aconductive backing member having a coating or layer of a photoconductiveinsulating material on at least one side. lf desired, thephotoconductive insulator may be protected by a thin iilm of aninsulating resin such as polystyrene, a cellulose ester, a polyvinylacetal, etc. The backing member may be of any conductive material havingthe desired structural properties for the use intended such as, forexample, a metallic member, plate or the like as of brass, aluminum,zinc, etc. or, optionally a nonconductive member of desired structuralproperties, such as glass or plastic having a conductive coating as oftin oxide, silver, etc., or a fibrous material, such as paper, havingtherein conductive material, such as water, particles of carbon, metalor the like, to render the material conductive. Representativephotoconductive insulators include vitreous selenium, sulphur,anthracene, mixtures of selenium and sulphur, various photoconductivephosphors such as zinc oxide, cadmium sulde and the like. These may beapplied to the conductive backing member either as a continuous layer oras discreet particles in a resinous binder.

The general nature of the invention having been set forth, `there willnow be presented a more detailed description, in illustration but notlimitation, of the invention in the following specication and drawingsin which:

Fig. l is a semi-diagrammatic representation of apparatus according toone embodiment of the invention, and

Fig. 2 is a diagrammatic view of development mechanism according toanother embodiment of the invention.

The present invention is particularly intended and adapted for thedevelopment of electrostatic latent images on relatively largexerographic plates such as are commonly used in the reproduction offull-size maps. Images of this sort require an unusually high degree offidelity and resolution in their reproduction.

Illustrated in Fig. l is a development system presentedsemidiagrammatically comprising a developer tray 10 movably positionedon slides 11 and 12 wherein it is yieldably secured by means 13, assprings or the like. Means 14 to vibrate the tray are also provided inthis case, consisting of an armature 15 and a solenoid 16 with a contactlever 17 which is loaded by spring 18. The slide and tray assembly isfree to rotate about axis made by the two slide means 11 and 12.

ln operation, a surface bearing an electrostatic latent image to bedeveloped is placed face up in the tray 10. When the image is on aphotoconductive insulating layer, as is the case when a xerographicplate is used, the entire unit must be handled in a darkroom to preventdissipation of the charge in the image areas. The image-bearing memberis secured to suitable means, as pins, to the developer tray 1b. Thetray is then positioned at a slight angle to the horizontal with asupply of developer, such as disclosed in 2,618,552, distributed alongthat end which is raised from the horizontal. The spring 18 is thenadjusted to obtain the desired rate of vibration and the vibrating means14 actuated by closing switch 19. Current ows through solenoid 16forcing armature 15 back against contact lever 17. This causes contactlever 17 to rotate on pin 21 breaking the circuit at contact point 20.With the force exerted by the armature 1 5 on assaasa 3 the upper arm ofcontact lever 17 removed by the breaking of the circuit at contact point20, spring 18 rotates contact lever 17 around pin 21 striking amature15. The upper arm of contact lever 17 drives armature 15 against traycausing' it to vibrate while simultaneously the lower varm of contactlever 17 restores current to coil 16 by making contact with contactpoint 20. The cycle repeats until switch 19 is opened. The time forcompleting one cycle may be adjusted either by varying the tension onspring 18 or by varying the point of contact between contact lever 17and contact point 20. The developer then cascades across theimage-bearing surface in a zigzag path and collects on the lower side ofthe developer tray 10. In this manner the image-is developed.

Example 1 The apparatus of Fig. 1 was used. The image to be developedwas on a Xerographic plate consisting of a layer of vitreous selenium onan aluminum backing and measuring 11 X 14" in size. The plate wassecured to the developer tray 10 by pins and the tray tilted about tendegrees to the horizontal. A two-component developer prepared inaccordance with U.S. 2,618,551 and available commercially from TheHaloid Company under the name XeroX Toner and Developer was distributedat the far end of the upper side of the tray. The armature weighed twoounces and was moved by a hundred-watt electromagnet. The armature 15traveled about onequarter inch. The spring 18 was adjusted to produce afrequency of about ten cycles per second. T he amplitude of vibration ofthe tray 10 was approximately 0.01. After the two-component developerwas distributed, the switch 19 was closed. Development was complete inabout one minute.

The image was then electrostatically transferred to a zinc plate usingthe apparatus of U.S. patent application Serial No. 491,344, tiled Marchl, 1955, by Matthews and Walker, with a potential of about 500 volts,and fused to the zinc plate in the same apparatus usingtrichloroethylene vapor. The image on the zinc plate was then examinedand found to be characterized by exceptionally good quality, having goodresolution, density, and relative absence of hollow lines.

In Fig. 2 the tray 30 contains a perforated screen 31 positioned in aslotted trough 32 at the upper end of the tray. The trough V32 connectsto the surface of tray 30 by means of slot 39. The tray 30 is fastenedto a tray support 33 which is fastened by yielding means as springs 34to stand 35. Stand 35, in turn, is positioned by rack 36 so that thetray 30 is inclined at an angle to the horizontal. A spout 38 is at thebottom of the tray 30. An eccentric 40 driven by motor 41 is connectedto tray support 33 by suitable means as a rod 42.

In operation, an image bearing member 44 having a photoconductiveinsulating layer 45 on a conductive back 46 is placed with theinsulating layer 45 up in the tray 30 where it is secured by suitablemeans, as pins 37 and ange 43. Alternatively, the tray 30 may be soconstructed as to firmly hold the image-bearing members to be developedtherein. A supply of two-component developer prepared in accordance withU.S. 2,618,551 Vand available commercially from The Haloid Company underthe name XeroX Toner and Developer is distributed `along the perforatedscreen in the slot developer trough.

The motor driving the eccentric is then actuated causing the tray tooscillate in the plane of the tray 30 and in a direction approximatelyat a right angle to the direction of motion of the powder particles overthe imagebearing member. The result is to impart a zigzag pattern to thedeveloper particles. These particles passing over the image-bearingmember pass through the hole 38 and yare collected in suitable means asa jar, trough, or other container. The .developer so collected may thenbe redistributed over the perforated screen for the development ofanother image-bearing member.

4 Example `2 The apparatus of Fig. 2 was used. The electrostatic latentimage to be developed was on a xerographic plate consisting of a layerof vitreous selenium on an aluminum backingV and measuring 11" X 14".Accordinglyfdevelopment wascarried out in the dark to prevent dissipa#tion of the electrostatic image. The plate `44 was placed in the tray 30image side (i.e., the `side bearing thephotoconductive insulating layer45) up. The stand 36 holding the tray 30 was so constructed that thetray was inclined at iive degrees to the horizontal. A supply oftwo-component developer prepared in accordancewith U.S. 2,618,551 andavailable commerically from The Haloid Company under the name XeroXToner and Developer was distributed Valong the length ofthe perforatedscreen 31 in the slot developer'trough 32. The eccentric 40 was adjustedto displace the tray 30 about one-eighth inch while the motor 41 wasadjusted to provide about thirty such oscillations per second. The motor41 Iwas then actuated. The two-component developer cascaded over theimage-bearing member 44 collecting at the bottom of the tray 30 where itpassed through hole 38 to be collected in a jar. Development took abouttwo minutes.

'Ihe image was then electrostatically transferred to a zinc plate usingthe apparatus of U.S. patent application Serial No. 491,344, filed Marchl, 1955, by Matthews and Walker, with `a potential of yabout '500 volts,and fused to the zinc plate in the same apparatus .usingtrichloroethylene vapor. The resulting image was characterized by goodresolution, density, and by'freedom from hollow lines.

Example 3 Example 2 was duplicated except that the stand 36 wasconstructed to incline the tray 30 at an angle of ten degrees to thehorizontal. In this case complete development of the 11" x 14"Xerographic plate was obtained in one-half minute. Again, the image onthe zinc plate `was characterized by good resolution, density, andfreedom from hollow lines.

While the present invention has been described herein as carried out inspecific embodiments thereof, it is not desired to be limitedthereby'but it is intended to cover the invention broadly within thespirit and scope of the appended claim.

I claim:

In the process wherein an electrostatic latent image on a smooth,planar, electrically insulating surface is developed to yield an imageof electrostatically attractable material electrostatically adhering onsaid surface by cascading across the electrostatic latent image a drymixture of loose, movable finely divided particles of electrostaticallyattractable powder and separate granular carrier material, theindividual granules of said carrier material being substantially largerVthan the individual particles of said attractable powder, the powderand carrier having a triboelectric relationship of opposite polaritywhereby the powder removably adheres electrostatically to the surface ofthe carrier granules the improvement comprising flowing the dry mixtureover the said image bearing surface ina zigzag course.

References Cited in the file of this patent UNITED STATES PATENTS238,991 Toye Mar. 15,1881 2,550,724 Sabel et al. May l, 1951 2,550,738Walkup May 1, 1951 2,618,552 Wise Nov. 18, 1952 2,635,046 Sahel et al.Apr. 14, 1953 2,682,478 Howse June 29, 1954 2,705,199 Clark Mar. 29,1955 Y 2,761,416 Carlson c Sept..4, 195.6

